Conventionally, there is an alarm device for calculating the parking angle of a vehicle with respect to the travelling lane, based on a yaw rate detected by a yaw rate sensor installed in the vehicle or the rudder angle detected by a rudder angle sensor for detecting the steering angle, storing the calculated parking angle in a non-volatile memory, etc., and setting the angle of an alert area behind the self-vehicle when reversing the vehicle from the parked state (see, for example, Patent Document 1).